Full Setup

Here's the full setup. I'm using a Raspberry Pi as the USB adapter, getting GND, and 5V out of it. The switchboard needs both 5V and GND, while the diode board only needs a ground, as they all light up on a positive signal. Seven 8-wide cables connect them, which is probably an overkill, but that's how many I could fit.

Diode Board

The diode board is made out of seven identical 8-bit parts. From bottom up:

8-bit signal input cable

8 diodes (colors rotating between red, green, and yellow depending on the group)

8-way 1k resistor array

Ground

The diodes were a bit too big to fit in line, but they actually look a bit better zig-zagging odd/even. I'm quite happy with this board.

Switchboard

This is a somewhat awkward build. There are also seven identical 8-bit parts. From bottom up:

5V

A lot of cabling to get it all together — I really hate this part of the build as it makes everything super awkward just to do something as simple as connecting a bunch of pins together. One upside of soldering is that these kind of connections are super trivial to do.

8-bit on/off switch

8-bit signal output signal cable

8-way 1k resistor array

Ground

So when the switch is off, the output is connected to a 1k resistor to the Ground.
And when the switch is on, the output is connected directly to 5V.

There are quite a few problems with this board.

Max power dissipation here is rather large, with 56 wires of 1k resistance each — that's the equivalent of just 18ohm, so that's 280mA or 1.4W just for one board. Using a PC power, supply that would be totally fine, but the USB max power supply is just 500mA or 2.5W, so we're already stretching it.

The most obvious solution would be to put much higher resistance, and a fancier way would be to do so on both the 5V and GND, and then use another board with a driver for output (like 74HC4050).

Another big problem is that switches are very hard to access. I basically need to use tweezers to flip them. The whole block of cabling on the bottom is extremely silly for what's basically "connect everything with everything" functionality.

Next Steps

I could definitely improve these boards, but they ought to be good enough for now. The next step would be figuring out a way to interact with the Raspberry Pi — I want it to be able to read signals from my switches and to send output to the diodes.

Raspberry Pi doesn't have anywhere near enough pins for it, so I might need some shift registers or other such chips to widen the interface.